A fourth generation (4G) mobile communications method which is the next generation of IMT-2000 (International Mobile Telecommunications 2000) is under development. The fourth generation (4G) method is expected to flexibly support various environments from a multi-cell environment including a cellular system to an isolated cell environment such as a hotspot area and an indoors area, and increase frequency utilization efficiencies in both cell environments.
In the fourth generation communications method, the following radio access methods have been proposed for a link from a mobile station to a base station (referred to as an up-link, hereinafter). As single-carrier transmission methods, a DS-CDMA (Direct Sequence Code Division Multiple Access) method, an IFDMA (Interleaved Frequency Division Multiple Access) method, and a VSCRF-CDMA (Variable Spreading and Chip Repetition Factors-CDMA) method have been proposed, for example. As multi-carrier methods, an OFDM (Orthogonal Frequency Division Multiplexing) method, a Spread OFDM method, an MC-CDMA (Multi-Carrier Code Division Multiple Access) method, and a VSF-Spread OFDM (Variable Spreading Factor Spread OFDM) method have been proposed.
The single-carrier method provides high power efficiency because peak power is lower in terms of consumption power in a terminal, which reduces back-off of a transmission power amplifier.
As an example of the single-carrier methods, the VSCRF-CDMA method is explained with reference to FIG. 1 (See patent-related document 1).
A spreading portion 1 includes a code multiplication portion 2, a repetitive synthesis portion 8 connected to the code multiplication portion 2, and a phase shift portion 10 connected to the repetitive synthesis portion 8.
The code multiplication portion 2 multiplies a transmission signal by a spreading code. For example, a multiplier 4 multiplies the transmission signal by a channelization code defined under a predetermined code spreading ratio SF. In addition, a multiplier 6 multiplies the transmission signal by a scramble code.
The repetitive synthesis portion 8 compresses the spread transmission signal in a time-wise manner and performs chip repetition a predetermined number of times (CRF times). The transmission signal to which the repetition has been applied presents a comb-shaped frequency spectrum. When the repetition number CFR is equal to one, the repetitive synthesis portion 8 has the same configuration and operations in the usual DS-CDMA method.
The phase shift portion 10 deviates (or shifts) a phase of the transmission signal by a predetermined frequency established specifically for each mobile station.
In the VSCRF-CDMA method, when the CRF is greater than 1, for example, equal to 4, a comb-shaped frequency spectrum utilized by each user is arranged in a distributed manner over the entire band, as shown in FIG. 2A. In this case, a user-specific frequency offset is smaller than an allocated bandwidth.
On the other hand, when CRF is equal to 1, the spectrum utilized by each user is arranged over a block, as shown in FIG. 2B. In this case, the user-specific frequency offset is greater than the allocated bandwidth.
In addition, there has been proposed a radio access method where a comb-shaped frequency spectrum in the frequency domain is obtained (See non-patent documents 1, 2).
A transmission apparatus 30 to which the radio access method is applied includes a FFT portion 12 to which a spread data sequence is input, a rate conversion portion 14 connected to the FFT portion 12, a frequency domain signal generation portion 16 connected to the rate conversion portion 14, an IFFT portion 18 connected to the frequency domain signal generation portion 16, a GI addition portion 20 connected to the IFFT portion 18, and a filter 22 connected to the GI addition portion 20, as shown in FIG. 3.
The fast Fourier transformation (FFT) portion 12 divides the spread data sequence every Q chips into blocks and performs a fast Fourier transformation, thereby transforming the blocks into the frequency domain. As a result, Q single-carrier signals are obtained in the frequency domain. By the way, the spread data sequence corresponds to an output signal of the multiplier 6 in the spreading portion 1 explained with reference to FIG. 1.
The rate conversion portion 14 repeats a predetermined number of times, for example, CRF times the Q counts of the single-carrier signals. As a result, the number of the single-carrier signals generated isNsub=Q×CRF. 
The frequency domain signal generation portion 16 shifts each single-carrier signal on the frequency axis so that the spectrum becomes comb-shaped. For example, when a process corresponding to CRF=4 is carried out, three zeros are arranged between every single-carrier signal. As a result, the comb-shaped frequency spectra explained with reference to FIGS. 2A and 2B are formed.
The IFFT portion 18 performs a fast inverse Fourier transformation on the comb-shaped spectra obtained by shifting each single-carrier signal on the frequency axis.
The guard interval addition portion 20 adds guard intervals to a signal to be transmitted. The guard intervals are obtained by replicating a portion of the top or end of a symbol to be transmitted. The filter 22 performs a band limitation on the transmission signal.
On the other hand, the multi-carrier method, which has a long symbol, can provide an improved reception quality in a multi path environment by providing the guard intervals.
As an example, the OFDM method is explained with reference to FIG. 4.
FIG. 4 is a block diagram of a transmission portion used in a transmission apparatus of the OFDM method.
The transmission portion 40 includes a series/parallel (S/P) conversion portion 32, a sub carrier mapping portion 34 connected to the S/P conversion portion 32, an IFFT portion 36 connected to the sub carrier mapping portion 34, and a GI addition portion 38 connected to the IFFT portion 36.
The series/parallel conversion portion (S/P) 32 converts series signal sequences to parallel signal sequences.
The sub carrier mapping portion 34 allocates to each sub carrier each signal which is converted to the parallel signal sequence in the series/parallel conversion portion 32. For example, the sub carrier mapping portion 34 allocates discrete sub carriers to each user as shown in FIG. 5A in order to obtain a frequency diversity effect. In addition, the sub carrier mapping portion 34 allocates consecutive sub carriers to each user as shown in FIG. 5B.
The fast inverse Fourier (IFFT) portion 36 performs the fast inverse Fourier transformation on the input signal so as to perform modulation of the OFDM method.
The guard interval addition portion 38 adds guard intervals to a signal to be transmitted and generates a symbol of the OFDM method.
Patent-related Publication 1: Japanese Patent Application Laid-Open Publication No. 2004-297756.
Non-patent Publication 1: M. Schnell, I. Broeck, and U. Sorger, “A promising new wideband multiple-access scheme for future mobile communication,” European Trans. on Telecommun. (ETT), vol. 10, no. 4, pp. 417-427, July/August 1999.
Non-patent Publication 2: R. Dinis, D. Falconer, C. T. Lam, and M. Sabbaghian, “A Multiple Access Scheme for the Uplink of Broadband Wireless Systems,” in Proc. Globecom 2004, December 2004.